The importance of energy-saving systems is growing due to such problems as environmental pollution and resource depletion, including global warming. With this trend, great attention has recently been given to the energy-saving effect of improving the power conversion efficiency of actual equipment which uses the energy of generated and supplied electrical power. The efficiency improvement of power conversion requires the performance improvement of switching devices used in the conversion circuit.
As such switching devices, power semiconductor devices are used. Thus, the performance improvement of power conversion circuits requires the performance improvement of power semiconductor devices. As the result of recent technology advancement, the performance of silicon power semiconductor devices currently in widespread use is reaching the limit imposed by the characteristics of the silicon material.
Against this background, research and development activities have recently been increased toward the performance improvement of power devices by the renewal of semiconductor materials. Gallium nitride-based materials are known as a strong candidate for such new materials.